Fix JSON in one of the examples

[tripu]

The 6 files under /spec/model/examples/ are not valid JSON.

While these are just examples, and not intended to be correct, breaking the JSON spec causes a few minor inconveniences:

• Syntax highlighting does not work on GitHub (example).
• Files are not parsed nor displayed as JSON on browsers (example).
• Some editors that detect JSON errors complain and highlight problems.
• As they are now, it's not possible to start using these examples programmatically (for early experiments, etc).

This PR “fixes” one of these examples. Validity can be checked eg using an online JSON validator/editor. Most browsers may display the file very differently, and editors won't complain. See the file on the browser (if your browser has a JSON extension, it will let you collapse/expand entire sections, highlight syntax, render URIs as hyperlinks, etc).

NB: inline comments that were deleted here will have to be put somewhere else (eg, a new text file just for this?).

@espinr, if you're happy about this change, I'll do the same with the other 5 files.

[tripu]

Notes to self:

1. Check:

   $ find spec/ -iname '*.json' | xargs -n 1 jsonlint-php

2. Fix quotes (if there are no legit occurrences of '):

   $ sed -i.bak "s/'/\"/gi" spec/model/examples/FILE.json

3. Remove comments.

4. Normalise formatting:

   const fs = require('fs');
   fs.readFile('spec/model/examples/FILE.json', (foo, data) => {
    fs.writeFile('spec/model/examples/FILE2.json', JSON.stringify(JSON.parse(data), null, 4) + '\n');
   });